Transmission line



Dec. 29, 1931. H. NYQUIST TRANSMISSION LINE Filed May 19, 1931 Yb Jen dz7/2 9 Filters atizer Chou/z neZs INVENTOR E/Yyquw fietector I2 15% 7 T25 Hecewmg 1 filters othel Chili/Heels ATTOR N EY Patented Dec. 29, 1931UNITED STATES PATENT OFFICE HARRY NYQUIST, OF MILLBURN, NEW JERSEY,ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION OFNEW YORK TRANSMISSION LINE Application filed May 19, 1931. Serial No.538,603.

This invention relates to transmission systems, and more particularly toa method of and means for obtaining separation between incoming andoutgoing signals applied to the lines of such systems.

A transmission line over which signals are being applied in bothdirections may be terminated at acentral office in a balancing networkand may include the windings of a hybrid coil. Associated with thewindings of the hybrid coil might be an incoming or receiving circuitand an outgoing or transmitting circuit. It is well known in the artthat if the balancing network balances the line perfectly, there wouldbe perfect separation between the incoming and outgoin signals and nointerference would result. flowever, in certain instances it may not bepractical from commercial or other standipoints to construct a networkwhich will substantially or exactly balance the transmission line, andthe primary object of the invention is to provide a means for obtainingthe desired separation between incoming and outgoing signals under suchinstances.

An illustration of an instance wherein the arrangements of the inventionwould be desirable would be in connection with a transmission line, suchas a loaded cable, overwhich carrier currents are utilized to transmittelegraph signals. The cable would terminate in a balancing network inthe usual manner, but it is probable that the ordinary network would notexactly balance the cable, principally for the reason that the cablemight not be uniform in structure and, as a result, there would bereflections from the various irregularities which would give rise to anunbalance current, which might result in imperfect separation betweenthe incoming and outgoing currents. An obvious way'of overcoming thisdrawback would be to modify the balancing network so that it wouldsimulate the line impedance. However, to do this over a large frequencyrange it is necessary to effectively duplicate theline by means of anartificial line or network, which simulates the real line in itspropagation characteristics as well as in its characteristic impedance.TVhile such a solution is possible, nevertheless it would lead to theconstruction of a very elaborate balancing network. The arrangements ofthe invention provide a solution of the problem which does not involvethe construction of an elaborate balancing network.

In the arrangements of the invention an auxiliary circuit is providedbetween the incoming and outgoing circuits connected to the transmissionline. The paths of this auxiliary circuit are interconnected by a hybridcoil having its windings balanced to the same degree as the windings ofthe hybrid coil in the transmission line. A condition of unbalance inthe auxiliary circuit is thus obtained which simulates the. condition ofunbalance between the incoming and outgoing circuits and the effect ofunbalance currents between said circuits will be prevented by theunbalance currents in the auxiliary circuit.

While reference has been had to cable circuits and to the use inconnection therewith of carrier currents for telegraph purposes, it ispointed out that this reference has been for purposes of illustrationonly, and that the arrangements of the invention are applicable to othertypes of transmission systems. Further objects and features of theinvention will appear more fully from the detailed description thereofhereinafter given.

The invention may be more fully understood from' the followingdescription, together with the accompanying drawing in the figure ofwhich is illustrated a preferred form of the invention.

In the drawing the invention is shown in connection with a transmissionsystem in which carrier currents are utilized for telegraph signallingpurposes. A transmission line L is shown terminating in a balancingnetwork BN and including the windings of a hybrid coil 1. Connected tothe mid-points of the windings of the hybrid coil would be atransmitting circuit 2 common to a number of individual transmittingcircuits. Only one of these circuits, namely the circuit 4, is shown indetail. The circuit 4 would include a filter SF and a circuit includinga source 5 of carrier frequency. The application of current from thissource to the line 1 would be controlled by a relay 6 and a telegraphkey 7. Connected to one of the windings of the hybrid coil is anincoming or receiving circuit 3 which is common to a number ofindividual receiving circuits such as 11. The circuit 11 would include afilter RF, a detector 12,. and a relay 13 associated therewith. Therelay 13 would control the operation of a sounder 14. Thearrangements-heretofore described are old in the art;

If the network BN exactly balanced the line L, currents transmitted fromthe circuit 2 to the line L would not affect the incoming or receivingcircuit 3. Furthermore, currents coming in over the line L would betransmitted to the circuit 3 and operate the receiving apparatus. Partof the current would also enter the circuit 2 and would be dissipated inthe sending circuits 4 and 9, only a negligible portion reaching thereceiving detector 12 through the hybrid coil 10. However, as has beenpointed out, aperfect condition of balance may not exist between thenetwork BN and line L, particularly if the line L is a circuit of thetype of a loaded cable. If a condition of balance does not exist betweenthe network BN and the line L, unbalance currents will arise in the windings of hybrid coil 1' and a perfect separation-between incoming andoutgoing signals willinot be attained.

In accordance with the arrangements of this invention, the apparatus tothe left of the dotted line AB is addedto the usual terminalarrangements of the line L. The circuit 4 would include a filter SFsimilar to the filter SF, and wouldbe connected to a circuit 9 connectedto the mid-points of the windings of hybrid coil 10. The circuit 11would include a filter RF similar. to the filter RF, and would beconnected to the circuit 17 connected to one of the windings of thehybrid coil 10. The windings of the hybrid coil 10 would be connected tothe balancing networks BN and BN The networks BN and BN would be madeequal to each other and one of them, such as BN would be built out bymeansof an auxiliary network, such as 21, so that a degree of unbalancebetween the windings of hybrid coil 10 would be caused to exist, whichwould simulate the degree of unbalance existing between the windings ofthe hybridcoil 1'. By this means the auxiliary path'between circuits 4;and 11 wouldbe so designed that the current transmitted through it wouldbe substantially equal to'and of opposite phase to the unbalancecurrent-arising from a-condition of imperfect balance between the line Land the balancingnetwork BN The current transmitted through thisauxiliary path would thus neutralize the effect of unbalance currentsarising in the windings of the hybrid coil 1.

Not only must the unbalance in the two paths be equal but alsotheattenuation, phase the one heretofore described would be ineludedbetween each individual transmitting circuit and its correspondingreceiving circuit.

While the invention has been disclosed as embodied in certain specificarrangements which are deemed desirable, it is understood that it iscapable of embodiment in many and other widely varied forms withoutdeparting from. the spirit of the invention as defined by the appendedclaims.

What is claimed is: 4

1. A transmission line terminating in a hybrid coil anda balancingnetwork, a transmitting circuit and a receiving circuit connected to thewindings of said hybrid coil, and an auxiliary circuit connecting saidtransmitting circuit with said receiving circuit, said auxiliary circuitbeing so designed that the currents transmitted therethrongh will besubstantially equal to currents arising due to a condition of unbalancebetween said transmission line and said balancing network.

2. A transmission line terminating in a hybrid coil and a balancingnetwork, a transmitting circuit and a receiving circuit connected tothe.windings of said hybrid coil, and an auxiliary circuit connecting saidtransmitting circuit with said receiving circuit, said auxiliary circuitbeing so designed that the currents transmitted therethrough will besubstantially equal to and will oppose the currents which might betransmitted fromsaid transmitting circuit to said receiving circuitthrough said hybrid coil due to a condition of unbalance'between saidtransmission line and its balancing network.

3. A transmission line terminating in a balancing network, transmittingand receiving circuits connected to said line in a condition approachingconjugacy, and an auxiliary circuit interconnecting said transmittingand receiving circuits, said auxiliary circuit comprising two pathsconnected to each other in a relation of balance substantially similarto the existing relation to balance betweensaid'transmitting andreceiving circuits.

4. A transmission line terminating in a hybrid coil and a balancingnetwork, transmitting and receiving circuits connected to saidhybridcoil, auxiliary circuits connected to said transmitting andreceiving circuits, and means for connecting said auxiliary circuitstogether, said last mentioned means comprising means for simulating acondition of balance between said auxiliary circuits similar to thecondition of balance existing between said transmitting and receivingcircuits.

5. A transmission line terminating in a hybrid coil and a balancingnetwork, transmitting and receiving circuits connected to the windingsof said hybrid coil, and an auxiliary circuit interconnecting saidtransmitting and receiving circuits, said auxiliary circuit including atranslating device whereby the current fiow in said auxiliary circuitmay be controlled.

6. A transmission line terminating in a hybrid coil and a balancingnetwork, transmitting and receiving circuits connected to the windingsof said hybrid coil, and an auxiliary circuit interconnecting saidtransmitting and receiving circuits, said auxiliary circuit comprisingtwo paths interconnected by a hybrid coil having its windings balancedto a degree similar to the degree of balance existing in said firstmentioned hybrid coil connection.

7. A transmission line terminating in a hybrid coil and a balancingnetwork, transmitting and receiving circuits connected to the windingsof said hybrid coil, auxiliary circuits connected to said transmittingand receiving circuits, a second hybrid coil having its windingsconnected to said auxiliary circuits, and balancing networks connectedto said second hybrid coil.

8. A transmission line terminating in a hybrid coil and a balancingnetwork, transmitting and receiving circuits connected to the windingsof said hybrid coil, auxiliary circuits connected to said transmittingand receiving circuits, a second hybrid coil having its windingsconnected to said auxiliary circuits, balancing networks connected tosaid second hybrid coil, and means for adjusting the constants of one ofsaid last mentioned networks.

9. A transmission line terminating in a hybrid coil and a balancingnetwork, a transmitting circuit and a receiving circuit connected to thewindings of said hybrid coil, a plurality of individual transmittercircuits connected to said transmitting circuit, filters in each of saidindividual transmitter circuits, a plurality of individual receivercircuits connected to said receiving circuit, filters in each of saidindividual receiver circuits, and auxiliary circuits connecting each ofsaid individual transmitter circuits with its corresponding individualreceiver circuit, and means in each of said auxiliary circuits forestablishing a condition of balance therein like unto the condition ofbalance existing between said first mentioned transmitting and receivingcircuits.

10. A transmission line terminating in a hybrid coil and a balancingnetwork, a transmitting circuit and a receiving circuit connected to thewindings of said hybrid coils, selective means in said transmitting andsaid receiving circuits, an auxiliary circuit interconnecting saidtransmitting and said receiving circuits, said auxiliary circuitcomprising two branches interconnected through the windings of a hybridcoil, means for balancing the windings of said hybrid coil to a degreesimilar to that existing between the windings of said first mentionedhybrid coil, and a variable attenuator included in one of the branchesof said auxiliary circuit.

11. A transmission line terminating in a hybrid coil and a balancingnetwork, a transmitting circuit and a receiving circuit connected to thewindings of said hybrid coils, selective means in said transmitting andsaid receiving circuits, an auxiliary circuit interconnecting saidtransmitting and said receiving circuits, said auxiliary circuitcomprising two branches interconnected through the windings of a hybridcoil, means for balancing the circuits including the windings of saidsecond hybrid coil to a degree similar to that existing between thecircuits including the windings of said first mentioned hybrid coil, avariable attenuator included in one of the branches of said auxiliarycircuit, and reactance devices in each of the branches of said auxiliarycircuit to balance the impedance of said selective means.

In testimony whereof, I have signed my name to this specification this18th day of May, 1931.

HARRY NYQUIST.

